Substantially all modern electronic computers rely on semiconductor memory to store data for processing by a central processing unit (CPU). Such computers employing semiconductor memory vary from simple computers, such as those contained in telephone answering machines, to highly complex supercomputers employed for complicated scientific projects. In simple computers like those used for telephone answering machines, errors in one or more of the memory locations of the memory may not be fatal. For example, a mistake in the memory of the telephone answering machine likely would only cause the synthesized voice stored on the memory to be imperceptibly altered. However, one or more defective memory locations in a memory of a computer used to perform scientific calculations may cause substantial problems.
Although current manufacturing techniques have substantially reduced the number of defective memory locations, computer memory still is susceptible to such defective memory locations. Those defective memory locations can be caused by any of numerous steps taken during manufacture of the memory chips, semiconductor crystalinity defects, electrical connector discontinuities, etc. Although memory chips with such defective memory locations typically represent a small portion (less than 1%) of the total number of memory chips produced, the actual number of such defective memory chips is substantial. In the past, extra rows of memory cells, known as "redundant rows" were provided that could be used to replace rows having defective memory cells. While the use of redundant rows is successful in salvaging otherwise defective memory chips, the number of defective rows that can be replaced is limited to the number of redundant rows that are provided on the memory chip. The number of defective rows sometimes exceeds the number of available redundant rows, thus preventing repair of some defective rows. In some cases, such defective memory chips could be sold at a greatly reduced price for applications that do not require perfect memory, such as for telephone answering machines. However, it would be beneficial if some of those memory chips could be employed in more critical applications, such as in personal computers.
One way to enable such defective memory chips to be incorporated into personal computers would be to employ error correction schemes to compensate for defective memory locations. Error correction schemes add to each data word plural error correction bits that enable the data word to be reconstituted in the event of an erroneous data bit within the data word. However, such prior art error correction schemes typically only reconstitute a data word if only a single bit of the data word is erroneous. Moreover, such error correction schemes add several extra data bits to each data word which results in high memory overhead. In addition, such error correction schemes could be extended to detect multiple erroneous data bits, but the memory overhead that would result likely would be unacceptable.